1. Field of the Invention
The present invention relates to a continuously variable belt transmission for a vehicle, and in particular to a continuously variable belt transmission that uses a belt to transmit motive force between two variable pulleys and, at the same time, control a gear ratio thereof by changing a winding radius of the belt.
2. Discussion of the Related Art
Generally speaking, a step variable transmission or a continuously variable transmission is provided on the output side of an engine with the aim of running the engine in an optimum condition according to vehicle operating conditions. There is, as an example of such a continuously variable transmission, a continuously variable belt transmission. The continuously variable belt transmission comprises two rotary members disposed in parallel with each other, and a primary pulley and a secondary pulley separately attached to each of the two rotary members. Each of the primary pulley and the secondary pulley is made up of a combination of a fixed sheave and a movable sheave and a V-shaped groove is formed between the fixed sheave and the movable sheave.
Further, a belt is wound around the groove in the primary pulley and the groove in the secondary pulley. Hydraulic chambers are separately provided, each independently generating a belt pressing force acting in an axial direction on the movable sheave. When the hydraulic pressure of each hydraulic chamber is independently controlled, the width of the groove in the primary pulley is controlled to vary the winding radius of the belt, thus changing its gear ratio. Meanwhile, the width of the groove in the secondary pulley is changed, so that the tension in the belt is controlled.
In a continuously variable belt transmission such as that described above, the hydraulic chambers are provided on an outer peripheral side of the rotary members. This possibly causes a hydraulic pressure generated by a centrifugal force or what is called the centrifugal hydraulic pressure, to act on the hydraulic chambers, making the hydraulic pressures in the hydraulic chambers higher than the controlled target levels. There is known the problem that this results in control accuracy of the groove width supporting the belt being degraded. Japanese Patent Application Laid-open No. 2001-323978 (corresponding to U.S. Pat. No. 6,565,465 B2) has described one example pertinent to a continuously variable belt transmission that can solve such a problem caused by the centrifugal hydraulic pressure.
In the continuously variable belt transmission disclosed in this publication, a secondary pulley provided on a secondary shaft is provided with a fixed sheave formed integrally on the secondary shaft and a movable sheave attached movably in an axial direction on the secondary shaft. A first hydraulic chamber is formed between the movable sheave and a partition wall, making the movable sheave be pressed in the axial direction, and a second hydraulic chamber is formed between the partition wall and a balance plate, providing with the movable sheave a pressing pressure in the opposite direction against the pressing pressure of the first hydraulic chamber. And an oil passage-forming member is located in a path of an oil passage communicated with the second hydraulic chamber. Further, the oil passage-forming member is attached to the secondary shaft, and a bearing and the partition member are provided on each side thereof, as well as an oil passage is formed in the secondary shaft where the second hydraulic chamber is communicated with the oil passage through a notch disposed in the oil passage-forming member. In addition, a compression coil spring is arranged in the first hydraulic chamber between the movable sheave and the partition wall, urging the movable sheave in the direction for generating a belt pressing pressure.
In a continuously variable belt transmission as described in the above publication, a centrifugal hydraulic pressure acts on the first hydraulic chamber while the hydraulic pressure in the first hydraulic chamber is being controlled and, even if the hydraulic pressure in the first hydraulic chamber becomes higher than a target pressure level, a hydraulic pressure corresponding to the centrifugal hydraulic pressure is generated in the second hydraulic chamber. As a result, the pressure corresponding to the centrifugal hydraulic pressure can cancel the centrifugal hydraulic pressure out. And since the movable sheave is urged in the direction so as to generate the belt pressing pressure by the coil spring, just in case a supply of an oil to the first hydraulic chamber becomes incapable, the belt pressing pressure is generated to prevent the belt from slipping even at the time a vehicle is towed, causing no problem with seizure of the belt.
The continuously variable belt transmission described in the above publication, however, has problems that the compression coil spring urging such a movable sheave is provided inside the first hydraulic chamber and is formed with a member independently of the balance plate defining the second hydraulic chamber, which makes the structural arrangement thereof more complicated and more costly.